1. Field of the Invention
This invention relates to an isocyanate prepolymer and a process for producing the same. More particularly, this invention relates to an isocyanate prepolymer having at least three isocyanate groups represented by the formula (I) above and an improved process for producing the same.
2. Description of the Prior Art
Hitherto, it is well known that compounds having a biuret structure represented by the formula ##STR4## wherein X represents a hydrogen atom or a ##STR5## group and R is an aliphatic, hydroaromatic, araliphatic including aralkyl or aromatic group which may or may not be substituted with Cl, NO.sub.2, an alkyl group, an alkoxy group or the like, can be prepared by reacting at least 3 moles of an organic diisocyanate with 1 mole of water in the absence of solvents, as disclosed in U.S. Pat. No. 3,124,605.
However, it was found that, although a diisocyanate generally reacts with water, a reaction system consisting of the diisocyanate and water temporarily forms an emulsion because of a low mutual solubility between the diisocyanate and water, and in such an emulsion system the compounds having the above biuret structure can only be produced by the reaction between the diisocyanate and water dissolved in a small amount in the diisocyanate phase. It was also found that the reaction between water and the diisocyanate dissolved in a small amount in the aqueous phase does not produce the desired compounds having a biuret structure but results in predominantly a polymer having the formula ##STR6## wherein R is as defined above and n is an integer of 1 or more, as a precipitate.
A polyisocyanate having a biuret structure is a liquid at room temperature (e.g. about 20.degree. to 30.degree. C.) and, by taking advantage of this, is useful as a raw material for urethane paint. However, the formation of a polyurea precipitate as a by-product associated necessarily with a polyisocyanate having a biuret structure adversely affects its utility and also makes the product remarkably less valuable. The polyurea is produced by the mechanism described above and generally is produced in an amount more than about 0.5% by weight based on the amount of the compound having a biuret structure, although the amount of the polyurea produced markedly varies depending upon the reaction temperature, the stirring rate used during the reaction, and the ratio of diisocyanate and water.
Of course, the amount of the polyurea produced as a by-product can be minimized if a large amount, i.e., more than 40 moles, of an diisocyanate is used per mole of water but the formation of the polyurea can not be avoided completely. Further, under such reaction conditions, a large excess of the diisocyanate must be separated from the compound having a biuret structure upon completion of the reaction and, therefore, a pure compound having a biuret structure can not be obtained economically.
Also, since the conventional process for producing a compound having a biuret structure is affected by the mutual solubility and the mutual solubilizing rate of the reactants in an emulsion state formed temporarily in view of its reaction mechanism as well as by the surface area of the emulsion phase, the molecular weight distribution of the compound having a biuret structure produced tends to be broad and, as a result, a compound having a biuret structure having a low molecular weight and a low viscosity can not be easily obtained.